Digital diversity communication systems make use of adaptive equalization techniques in order to remove as efficiently as possible intersymbol interference and to avoid, or at least considerably reduce, the possibility of errors due to the presence of noise. A typical adaptive receiver system using equalizer circuitry, as in the form of an adaptive transversal filter circuit, sometimes referred to as a tapped-delay line equalizer circuit, is discussed, for example, in U.S. Pat. No. 3,879,664 entitled "High Speed Digital Communications Receiver", issued to P. Monsen on Apr. 22, 1975. In such a system, adaptive weights W.sub.0. . . W.sub.n are used at each of the delay line taps, the weights being selected so that intersymbol interference is markedly reduced, while at the same time increasing the noise level as little as possible.
The general theory of the operation of such circuitry is now well-known to the art. Thus, the operation thereof requires that some values of the polarity of the transmitted signal be known to the receiver, which values are used as a "reference" signal. In one case, for example, such polarity values are provided by inserting identifiable pulses of known polarity in the transmitted data stream, in which case the equalization circuit is often designated as a "transmitted reference" equalizer. In another exemplary case it is possible to assume that nearly all of the received pulses out of the equalizer circuit have the correct polarity and these output polarities are used in the calculation of the weights. Such an equalization circuitry is often referred to as a "decision-directed" equalizer.
An error signal occurs when the output of the equalization circuit differs from the reference signal, the weight values used in such case in the equalizer circuitry being incorrect. A feedback circuit responsive to the error causes the weight values to be appropriately changed so as to reduce the error. One of the problems in achieving adequate performance of the equalizer circuit occurs when there are relatively rapid fluctuations in the intersymbol interference level, in effect producing fluctuations in the input to the equalizer circuit. Such fluctuations are often caused by imperfect operation of the automatic gain control (AGC) circuitry used in the receiver system or by other pre-processing of the received signal, one example of which is the use of adaptive matched filter technology used with the equalizer circuitry. If the time constant of the overall feedback path is too long, the circuit can not track such rapid fluctuations in input level in a manner well enough to achieve the desired weight values required so that leakage of intersymbol interference occurs. On the other hand, if the time constant is too short, there will be excessive noise fluctuations of the weight values about their desired values, thereby also causing undesired leakage of intersymbol interference.
It is normally more effective to use a time constant that is sufficiently long that excessive noise fluctuations do not occur in the computed weight values. Such operation, however, may in some cases fail to track signal input level fluctuations well enough to assure that the correct weights are achieved when rapid input level fluctuations occur.
It is desirable, therefore, to devise an appropriate technique which will better assure that the weight values which are computed for use in the equalizer circuit are substantially correct even when rapid fluctuations in the input levels to the equalizer circuit occur, while at the same time providing operation which does not appreciably increase the noise level effects in the equalizer operation.